Field of the Invention
The invention relates to an electronic circuit with a driver circuit to drive a signal onto a signal line.
In radio-frequency bus systems, reflections of signals on line ends, line divisions and the like result in unwanted overlays of the original signal with a reflected signal. The original signal is thereby amplified or attenuated. Both outcomes produce an unwanted, uncontrollable signal change. An interface which drives a signal onto a bus line or which receives the signal from the bus line is often located at one line end of a two-way bus system. When a signal is received, one unit, which normally carries out the signal drive during transmission, is switched to a passive mode. When a signal is transmitted, on the other hand, the receive component of the interface is disabled or deactivated.
Particularly when the signal is received via the bus line, the original signal may be overlaid by reflected signals occurring on an unterminated bus line end. The resulting change in the original signal may cause transfer errors.
So that essentially only the original signal is read when the signal is received, terminating resistances are provided on line ends, representing a sink for the incoming signal, whereby the incoming signal is not reflected. The terminating resistance is dimensioned in such a way that it corresponds to the characteristic line impedance.
However, the disadvantage of providing a fixed terminating resistance on a line end is that, when a signal is driven by a driver stage, some of the power of the transmitted signal is lost via the terminating resistance. It is thus necessary to increase the dimensions of the driver stage more than would be necessary without a terminating resistance.
It is accordingly an object of the invention to provide an electronic circuit with a driver circuit that overcomes the above-mentioned disadvantages of the prior art devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, an electronic circuit. The electronic circuit contains a first supply voltage terminal, a second supply voltage terminal, and a driver circuit for driving a signal onto a signal line having a specific characteristic impedance. The driver circuit has a first switching device with a first forward resistance between the first supply voltage terminal and the signal line, and a second switching device with a second forward resistance between the second supply voltage terminal and the signal line. A control circuit is connected to the driver circuit and generates a first control signal and a second control signal for controlling the first switching device and the second switching device in a first operating mode such that, depending on the signal to be driven, either the first switching device or the second switching device is through-connected so that the signal will be driven onto the signal line. The control circuit activates the first switching device and the second switching device substantially simultaneously in a second operating mode with an aid of the first and second control signals so that the first and second forward resistances together form a terminating resistance.
According to the invention, the electronic circuit is provided with the driver circuit to drive the signal onto the signal line. The signal line has a specific characteristic impedance. The driver circuit contains the first switching device with a first forward resistance between a first supply voltage terminal and the signal line, and the second switching device with a second forward resistance between a second supply voltage terminal and the signal line. The control circuit is furthermore provided in order to generate the first and second control signals. In the first operating mode, the first and second switching devices are controlled by the first and second control signals in such a way that, depending on the signal which is to be driven, either the first switching device or the second switching device is through-connected so that the signal is driven onto the signal line. In the second operating mode, the first and the second switching devices are activated substantially simultaneously with the aid of the first and second control signals so that the forward resistances of the first switching device and the second switching device together form a terminating resistance.
The circuit according to the invention therefore provides that the first and second switching devices that are normally provided in order to construct a driver stage are used in order to form a terminating resistance when the driver circuit is not required, e.g. when a signal is received. As a result, on the one hand, no additional terminating resistance needs to be provided which reduces the signal power when a signal is transmitted, and, furthermore, the terminating resistance can be accurately set by dimensioning the first and second switching devices.
According to a further preferred embodiment of the invention, the first switching device has one or more parallel-connected first switching elements, to which the first control signal is in each case applied, and the second switching device has one or more parallel-connected second switching elements, to which the second control signal is in each case applied. At least some of the first switching elements can be activated in each case via one of the first selection signals, and at least some of the second switching elements can be activated in each case via one of the second selection signals. In this way, the first forward resistance can be set by the forward resistances of the first switching elements activated with the aid of the first selection signals and the second forward resistance can be set by the forward resistances of the second switching elements activated with the aid of the second selection signals.
The plurality of first and second switching elements can be enabled or disabled so that, in the second operating mode, the first and second forward resistances can be accurately set. The first forward resistance and the second forward resistance are derived from a parallel connection of the forward resistances of the switching elements activated by the selection signals. From the perspective of the incoming signal, the terminating resistance then corresponds to a parallel connection of the first and second forward resistances. The first and second selection signals can be provided in the first operating mode in such a way that the driver power of the driver circuit can thus be set. The greater the number of switching elements activated by the selection signals, the higher the driver power will generally be.
The first switching elements may in each case have a first transistor, whereby the first control signal is applied to a control input of the first transistor depending on the corresponding first selection signal. One of the second switching elements has a second transistor, whereby the second control signal is applied to a control input of the second transistor depending on the correspondingly allocated second selection signal. A first NAND circuit is preferably connected to the control input of the first transistor and a second AND circuit is preferably connected to the control input of the second transistor. The first control signal is applied to a first input of the first AND circuit and the corresponding first selection signal is applied to the second input of the AND circuit. The second control signal is applied to a first input of the second AND circuit and the correspondingly allocated second selection signal is applied to the second input of the AND circuit. The simplest possible configuration is achieved in this way for the first and second switching devices with switching elements, whereby the forward resistance is essentially formed by the forward resistances of the first and second transistors. Generally, these can also be accurately defined in an integrated process by setting process parameters.
According to a further aspect of the present invention, it can be provided that the first switching device contains a plurality of parallel-connected first transistors and a series-connected second transistor, and the second switching device contains a plurality of parallel-connected third transistors and a series-connected fourth transistor. The first control signal is applied to a control input of the second transistor and the second control signal is applied to a control input of the fourth transistor. The first selection signals are applied to the control inputs of the first transistors and the second selection signals are applied to the control inputs of the third transistors. The first forward resistance can be set by the forward resistances of the first transistors activated with the aid of the first selection signals and the second transistor, and the second forward resistance can be set by the forward resistances of the third transistors activated with the aid of the second selection signals and the fourth transistor. In this way, a driver circuit configured with the smallest possible number of transistors can be implemented, whereby the driver circuit is suitable for providing a terminating resistance for the connected bus line which can be set as accurately as possible in the second operating mode. In the first operating mode, the driver power is defined by the first and third transistors that are activated by the first selection signals and the second selection signals.
In order to achieve the best possible termination of the signal line, it can be provided that the first and second selection signals are selected in such a way that the terminating resistance corresponds to the characteristic impedance of the signal line.
It can furthermore be provided that the first selection signals and the second selection signals are selected in order to define the first forward resistance of the first switching device and the second forward resistance of the second switching device in such a way that the signal line has a predefined voltage level. The voltage level on the signal line preferably lies midway between a first voltage level on the first supply voltage terminal and a second voltage level on the second supply voltage terminal.
It can furthermore be provided that the control circuit has a data input for applying data and a termination control input in order to apply a termination control signal. In addition, the control circuit has a NAND circuit and a NOR circuit. The data signal is applied to a first input of the NOR circuit and the termination control signal is applied to the second input of the NOR circuit. The first control signal is present at the output of the NOR circuit. The data signal is applied to a first input of the NAND circuit and the termination control signal inverted via an inverter is applied to a second input of the NAND circuit. The second control signal is then present at each output of the NAND circuit. The control circuit can thus be implemented in a simple manner, whereby the changeover between the first operating mode and the second operating mode is indicated by the termination control signal. If the termination control signal indicates that the driver circuit is to be switched to passive, the first control signal and the second control signal are switched in such a way that the first switching device and the second switching device are through-connected, and their forward resistances produce the terminating resistance.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an electronic circuit with a driver circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.